The present invention relates generally to metal-halogen battery assemblies and more particularly to a technique for improving the operation of a metal-halogen battery assembly of the rechargeable type such as the assembly disclosed in Applicant's copending U.S. Patent Application Ser. No. 841,391 which was filed Oct. 12, 1977 now U.S. Pat. No. 4,162,351 and which was assigned to the Assignee of the present application.
Applicant's copending application just recited discloses an overall metal-halogen battery system of the rechargeable type and specifically to an improved technique for reversibly complexing the halogen in such systems, particularly in a zinc-bromine system. This system, as described in the application, includes a positive electrode (the bromine electrode) and a negative electrode (the zinc electrode) which are respectively located within two sections of a housing or cell chamber appropriately divided by a suitable separator. In order to minimize the formation of zinc dendrites, the particular system disclosed utilizes continuously circulating aqueous electrolyte solution which contains bromide ions. Moreover, because the electrolyte on the negative electrode side of the separator must typically have a much lower bromine concentration than the electrolyte on the positive electrode side, two separate flow loops have been provided, the electrolyte in each loop making many passes through its respective chamber section during a single charge-discharge cycle. For a number of reasons described in the copending application, this battery system utilizes a complexing agent for adding bromine to the positive electrolyte solution circulated through the positive chamber section. The bromine is added by means of a polybromide oil phase which is located outside the battery chamber.
While the battery system described in copending application, Ser. No. 841,391 is satisfactory in most respects, Applicants have discovered room for improvement. For example, in the system described, a supporting electrolyte, specifically potassium chloride, is preferably used. Potassium chloride has a strong effect on the apparent transference number of the zinc species. Specifically in an acidic solution of concentrated chloride ions, the zinc ions are predominantly in zinc-polychloro complexes (ZnCL.sub.4.sup..dbd.. Since the net charge on these ions is negative, their direction of migration in an electrical field is opposite to that of an uncomplexed cation and hence they migrate through the separator from the negative electrolyte to the positive electrolyte during cell charging, thereby resulting in a depletion of the zinc ion concentration on the negative side. At the same time, the bromide ions behave as anions, and thus also migrate from the negative side to the positive side during the charge cycle. Moreover, the bromine partition properties of the complexing agent used to store bromine in a second liquid phase, specifically the polybromide oil, are poorer in high bromide ion concentration which Applicants have found specifically exists in cells with equal distributions of electrolyte on each side of the separator, that is, in each separate loop.
As will be described in detail hereinafter, in order to overcome the drawbacks just recited, Applicants have found that the overall system should be designed so that the negative electrolyte loop includes more electrolyte solution by volume than the positive loop and preferably as large a fraction as possible of the total electrolyte solution. This has several advantages. First, most of the zinc ions are already on the correct side, that is, the negative side, and migration to the positive side during charge will result in a more rapid concentration on the positive side, thereby enhancing back-diffusion to the negative side and increasing its fraction of utilization. At the same time, the bromide ion concentration on the positive side during the charging cycle will be depressed, resulting in better performance of the complexing agent with respect to the bromine partition. On the other hand, during the discharge cycle, the bromide ion concentration will be large, which will drive the bromine into the aqueous phase at an enhanced rate, which is desirable for efficient discharge. To these ends, the volume of electrolyte on the positive side would ideally be just enough to fill the plumbing plus a small surge volume, that is, just enough to support circulation through the positive loop.